The present invention relates to a codec(coder/decoder) for encoding/decoding binary value data, i.e., image data obtained from a document in an image transmission system such as a facsimile machine, and more particularly to a changing pixel detector having a simplified structure and an improved encoding function.
Generally, an image transmission system, such as a facsimile machine, for transmitting data of document processes binary value data using MH(Modified Huffman: one-dimensional run-length encoding) and MR(Modified Read: one-dimensional and two-dimensional encoding) methods recommended by CCITT International Telegraph and Telephone Consultative Committee. The encoding according to the MH and MR methods is performed in units of line, and the binary value data is compressed based on position information of brightness changing pixels in each line. The MH method is known as a run-length coding used to generate a predetermined code word in a code word table according to white run and black run of binary value data. The MR method is for encoding data in one of vertical, horizontal and pass modes according to correlation of the brightness changing pixels between a previous line and a current line of data as recommended by CCITT T.4.
A codec adapted in an image transmission system to perform the encoding/decoding becomes ASIC Compatible because it has smaller size and lighter weight. Conventionally, a full hardware codec has been used in a public switched telephone network(PSTN). In this case, the speed of the encoding/decoding is higher than the transmission speed when the encoding/decoding is performed by the full hardware using a central processing unit having a slower processing speed. But, since Group III facsimiles need to keep a minimum scanning line time and a fixed transmission speed, the encoding/decoding speed has little advantage, compared to the case of having the equal transmission speed. Also, when the full hardware codec is used, there still exists the high cost of the whole system due to added hardware.
Meanwhile, if the MH and MR is performed by software, the encoding/decoding speed is slower than the transmission speed of encoded data through the PSTN, and accordingly, the transmission time becomes longer. Here, the reason for a longer transmission time is that the transmission side should transmit dummy data during the speed difference time.